The present invention relates to a device for separating sheets from a sheet stack in general, and more particularly this invention relates to a sheet-separating device for separating printed sheets from a stack by means of cam-controlled or pneumatically controlled suction devices in paper-processing machines, for example printing machines.
One of conventional sheet-separating device has been disclosed in German patent DD PS No. 80 719. The known device includes a roller supported in the stationary frame of the device and operated to drive, via a roller lever, a four-link power transmission. The suction head in this known device is movable by the four-link transmission such that a curved movement of the suction head is approximately adjusted to a vertical; in other words an approximately uniform angular adjustment of the suction head to the upper surface of the paper to be separated from the stack has been ensured. The disadvantage of this known sheet-separating device is that each sheet to be removed, and particularly the ridge formed at the rear edge of the stack becomes adhered to the following sheets thereby preventing the normal separation of the sheets from the stack because each sheet can cause a lifting movement and an insignificant horizontal movement of the following sheet in the stack. Despite the utilization of various known rubbing-off elements, such as wipers or the like, double-sheets have been constantly separated from the stack. The adjustment of the position of the suction head to the upper surface of the stack has been obtained in the known device by an additional double-parallelogram suspension of the suction head so that the adjustment of the angular position of the suction head about an adjustable pivot point on the stationary frame could be achieved. The once adjusted angular position was maintained but not for the entire working stroke uniformly and thus not for the entire sheet-separating process.
Another sheet-separating device is disclosed in German patent publication DE-AS No. 24 23 349. This known device enables an angular adjustment of the suction head in the sheet-conveying direction and also in the transversal direction. The suction nozzle is movably positioned in the longitudinal direction of the sheet stack and is adjusted relative to the vertical axis by an adjusting screw engaged with and releasably supported on the upper end of the suction head by a knurled nut. The suction head thereby pivots over a predetermined curved path and the adjustment of its vertical position results to align the sheets, which lie non-parallel to the guide rails during the movement of the suction head. The respective adjustment of the horizontal surface of the suction head to the upper surface of the sheet stack results in this construction in that the sheets arrive at the conveyor marks in different positions. A combination of various cyclic movements in one stroke interval has been, however non-realizable in this conventional device.
Still another known device of the type under consideration has been described in German patent DD-PS No. 42 850. A reliable sheet-separation has been accomplished in this known device by providing a sudden tilting movement of the suction head about the edge of the suction head and by a subsequent impact-like lifting of the suction head from the stack. These movements, however could not provide for desired results in separating sheets from the stack because the impactwise movements of the suction head, effected by pneumatically-operated elements, caused the suction effect which led to pulling off the following sheet together with the sheet being separated. The adjustment of the suction head to the existing upper surface of the stack during the operation of the printing machine has not been, however, possible.
The German patent DD-PS No. 717, 654 discloses a movable-up- and down carrier, at the lower end of which a cam-controlled four-link transmission is provided, having a coupling member connected to the sheet-separating suction head. The adjustable coulisse guide imparts to the coupling member and to the suction head therewith, an adjustable tilting movement. The horizontal movement of the suction head is avoided in this known device in order to prevent sheet pulling during the tilting movement of the suction head.